Graffiti has been an eyesore for centuries. The advent of spray paint and felt tip markers has facilitated the defacing of property. Recently, a new form of defacing property has emerged. Graffiti vandals scratch and embed their names, initials and various insignia into various glass and plastic surfaces. This is accomplished using sandpaper, emery cloth, silicon carbide cloth, paper clips, knives or other hard substances that make penetrating scratches.
Some of the types of surfaces most often defaced are storefront windows, bus windows, train windows, and bus-shelters. Repair of the affected surfaces can be costly. Currently, the only available practical solution is replacing the glass or plastic window material. This is time-consuming and expensive because as soon as the window material is replaced, it is often defaced again.
The type of glass typically used on buses and trains is tempered glass. A panel of tempered glass used on a car of a train, which may measure 2 feet by 3 feet, can be expensive. The polycarbonate plastic of choice used on buses, trains and bus shelters is also expensive.
In the locations where the surface has been scratched, the surface turns white. This is due to the scattering of light. In addition, the depth of the scratches can vary. Replacement of the glass and plastic therefore becomes a necessity. If the window material is not optically clear, whitening caused by scratches can create an unsafe condition; people may not know which direction to exit a bus or a train in an emergency. In addition, people cannot clearly see street signs and other landmarks when they cannot see through the scratched windows of a bus or train.
Removal of scratches in situ is difficult. Sanding plastic and glass material until the base of the scratch is reached and polished is very difficult and time consuming. Often the labor cost exceeds the replacement cost of the glass or plastic.
In order for a surface coating to be distortion-free, it should have the same (or very close to the same) refractive index as the substrate on which it is applied. The refractive index is a measure of the angle that light is bent when it passes through a surface. Tempered glass and polycarbonate have different refractive indexes. The refractive index of glass varies from 1.517-1.890 using sodium vapor as the light source. The refractive index of polycarbonate is 1.586. It is difficult to find a single surface coating that may be applied to both surfaces and remain distortion free because their refractive indices are so different.
Whitening of a plastic or glass surface when scratched or damaged is caused when material is removed and displaced onto the surface of the scratch. Both the scratch and the material on the surface scatter light.
Adhesion of a coating to a substrate can be a significant problem. Glass and polycarbonate are relatively non-porous. It is difficult for a coating to bond well to non-porous surfaces. It is difficult to find a single surface coating that adheres well to both glass and plastic surfaces since the two have different chemical and surface properties.
Finding a suitable surface coating to remove and/or repair these scratches is a challenge. The perfect coating should: fill in the scratches, remove the surface whitening, leave the glass or plastic optically clear, be hard, chemically non-reactive, dimensionally stable, non-yellowing, recoatable and have good surface adhesion.